A co-culture consisting of Clostridium thermocellum ATCC 27405 and Thermoanaerobacterium thermosaccharolyticum DSM 571 was employed to improve the ethanol yield from microcrystalline cellulose (MCC) and corn straw substrates. An ethanol concentration of 1.29 g/L (26.1% ethanol yield) was obtained with 98.6% cellulose degradation when MCC was used as substrate in fermenter tanks. The ethanol yield obtained in fermenter tanks was 13.9% higher than that obtained via an anaerobic process performed in bottles. An ethanol concentration of 0.45 g/L, corresponding to 55.6% cellulose degradation and 11.2% ethanol yield, was achieved with corn straw as substrate in fermenter tanks. This ethanol yield was 28.2% higher than that formed in anaerobic bottles. Surprisingly, a 40.7% hemicellulose degradation was achieved via fermentation tanks, which was 127% higher than that obtained from anaerobic bottles.